Consider difference between MMO traits/gear (permanent, or at least until something better comes along) and FPS traits/gear (much more disposable).

In persistent games, you may not be able to balance as you go post-release without screwing your players, so balance early and often pre-release.

Large groups of small changes paired with tiny benefits (sugarcoating the pill) may alleviate player anger with key-jingling. "The new version has puppies! and your warrior is nerfed a lot but it's okay because PUPPIES!" beats "We slightly nerfed you today, and why not?"

Ask yourself when balancing post-release: is a change worth player backlash? A minor change may still engender major anger, as players gets attached to their gear and strategies.

If you think "they're playing it wrong," it's your problem - not theirs. It's more important that the players are enjoying themselves than it is that their playing style fits your vision.

Vocally angry players may represent only a mistaken minority, but they can poison other players who wouldn't have thought anything was wrong with the game.

In persistent worlds, balance changes can be excused through good storytelling.

Keep in mind, pre-release playtesters may WANT to find something wrong, whereas post-release players are more like to just enjoy themselves unless something pops out at them.

Good metrics can ID problems. Ask: where did deaths occur on this FPS map, where were they shot from, with what weapon? Or, where were goals scored from in this soccer/hockey game? Any trends you find may indicate an unfair advantage. (I personally used the same tactic over and over in NHL Hockey on Game Gear, scoring about 90% of my shots)